Synergistic chelate combinations in dilute immersion zincate solutions for treatment of aluminum and aluminum alloys



United States Patent ()flice 3,216,835 Patented Nov. 9, 1965 SYNERGISTICCHELATE COMBINATIONS IN Dl- LUTE lMMERSlON ZINCATE SOLUTIONS FORTREATMENT OF ALUMINUM AND ALUMINUM ALLOYS Edward ll. Saubestre, Hamden,Conn., asslgnor to Enthone, incorporated, New Haven, Conn., acorporation of Connecticut No Drawing. Filed Oct. 6, 1960, Ser. No.60,795 19 Claims. (Cl. 106-1) The present invention relates to improvedimmersion zincate solutions for the treatment of aluminum and aluminumalloys.

Aluminum and its alloys are difllcuit to plate because of the rapiditywith which they form an oxide coating when exposed to air. As a result.special treatments must be employed when plating on aluminum. Thesetreatments include mechanical treatments: chemical etches. especiallyacid etchcs containing iron. nickel, and manganese salts; alkalinedisplacement solutions, especially those depositing zinc. brass, andcopper: anodizing. especially in phosphoric, sulfuric or chromic acids;electroplating with zinc at low current densities for a few seconds. Ofthese treatments, the alkaline displacement solutions are generally themost successful commercially. in a summary by W. O. Zelley (J.Electrochem. Soc. 100, 328-332 (1953)), it is pointed out that suchsolutions are the most practical and economical of the various processesmentioned above. in general, alkaline displacement solutions requireless time and equipment. are less critical to control, and areapplicable to a wider range of alloys and shapes of parts.

While many metals can be deposited on aluminum by displacement, zinc isthe most common. in this case. the process is known as the zincateprocess.

CONCENTRATED ZiNCATE FORMULAS The conventional zineate solution isusually quite eoncentrated (on the order of g./l. of total salts) andusually contains sodium hydroxide and zinc oxide. The NaOH concentrationmay advantageously be as high as 800 g./l. since higher NaOliconcentrations generally yield thinner and more uniform zine films whichis advantageous for subsequent plating operations. At temperatures of-27 C. the immersion is for A-l minute. This yields a film weight of l-7mgJdmJ, depending on the alloy. Aluminum alloys H00 (28), 3000 (3S).2024 (248), 5052 (528) and 606i (618), as designated by ReynoldsAluminum Data llooit. published by Reynolds Aluminum Company, Richmondl8. Virginia, were subjected to this experiment. Best results (mostuniform film) are obtained by removing the zinc coating in a brief dipin nitric acid, then re-applyiag the zinc coating a sec 0nd time. Thisis the so-called "double zlneate" process. and is widely usedcommercially.

Since the formulas above result in an exceptionally high alkalinity,some workers have preferred to use milder aikalis, while retaining theuse of very concentrated solutions to obtain thin, uniform zinccoatings. Examples of such milder alkalls are carbonate and tartrate.

AC'iiVATED ZlNCATE FORMULAS During the years, a number of improvementshave been made in the conventional zineate formulation. most of themaimed at accelerating the rate of film formation. and the degree ofadhesion and uniformity of the zinc coating produced. Two approacheshave been used: the addition of an activating metal ion. or the additionof complexing agents for aluminum to hasten the rate of removal of theoxide film on aluminum.

till

mums ZINCATE FORMULAS All of the formulas referred to above involveconcentrated solutions (on the order of 500 g./l. of total salts). inaddition, the literature contains some reference to more dilute baths.For purposes of convenience, concentration ranges may be classed asfollows:

(a) Conventional: Total salts of 3 lb./gal. (350 g./l.)

or more (b) Moderately dilute: Total salts of between 1 and 3 lb./gal.(between 180-360 g./l.)

(c) Dilute: Total salts of 1% lb./gal. or less (180 g./l.

or less) An example of a moderately dilute bath is:

G./l. NaOH ZnSO -7H O 95 NuCN 40 Ntl SO.

it will be noted that the moderately dilute formula given above callsfor a higher percentage of complexing agent (cyanide) than that presentin conventional or activated conventional formulations. Moderatelydilute formttlatioas may also be of the metal activated type:

The above was particularly recommended for plating on copper-siliconaluminum alloys. Among the dilute formulations falling under theprevious definition, very few are simple zincatlag baths.

SPECIAL PROCEDURES Some special procedures have been published whichinvolve zineating as an essential step in the preparation of aluminumfor subsequent finishing. For example, in one method. the work is firstdipped in an alkaline iron solution until it gets dark; the coating isthen stripped and the work immersed in a zlncate solution.

in accordance with the present invention, 1 have discovered new andgreatly improved formulations for dilute zincatiag baths. By theprevious definition. this refers to baths whose total salt content ispreferably g./l. or less. Dilute zlncate baths have the followingadvantages over conventional zincating formulations:

(a) The lower concentration results in much lower viseosities. Thisminimizes trapping of solution in porous die castings and the like. Suchtrapped solution is a source of blistering of subsequentcicctrodeposlts.

(b) The resulting lower viscosity leads to lessening drag-out lossesfrom operating tanks. This can be a sizeable economic factor wheredrag-out is high. as when the shape of the parts being plated is such asto trap substantial quantities of solution.

(c) The more dilute the bath. the lower the initial cost of bath make-uptends to be.

(d) The more dilute zincate baths tend to operate faster thanconventional baths. This is an advantage in the operation of fullyautomated plating installations. it

is also advantageous when plating alloys which tend to form surfaceoxides rapidly. in the latter case it is necessary to form the zinccoating as rapidly as possible.

Despite the above advantages, dilute zincate formulations have foundrelatively little commercial application compared to conventional,concentrated formulations. The reason is that the dilute zincate bathshave been hard to control with resulting tendency to formation ofblisters after subsequent clcctrodcposits have been applied. Inaddition, any given dilute zincate bath has tended to work for only alimited number of alloys of aluminum. Thus, if a variety of alloys hadto be processed, no one given formulation could be reliably utilized.

The purpose of this disclosure is to reveal new formulations for dilutezincate baths which overcome the above disadvantages of dilute baths,while retaining all of their advantages.

The principal shortcoming of previous dilute zincate baths is that thedisplacement deposit of zinc tends to form so rapidly as to be ofunsound structure. Thus, such baths can be used only on some aluminumalloys which are suitable for such baths. Aluminum alloys containingmagnesium and silicon, for example, cannot be treated reliably in suchformulations. Previous atlcmpts to remedy these defects have centered onthe use of metallic activators, such as iron or copper or even lead, oron the use of complcxlng agents such as cyanide. Another approach hasbeen to add small amounts of inorganic oxidizing agents, such as nitriteand/or nitrate.

None of these attempts to overcome the objections to dilute zincateformulations have been fully successful. in particular, the aboveformulations often cause blistering after subsequent electroplating.Specifically, dilute zincate baths containing cyanide have not beennearly as successful as conventional zincating baths.

1 have found that all of the above objections may be overcome by usingdilute zincate formulations containing no complexing agents for zinc oraluminum other than hydroxide and to which chclating agents have beenadded which have the following properties, either singly, or insynergistic combination:

(a) Capable of chclating both aluminium and zinc.

(b) Capable of chclating strongly enough to prevent an unsound zincdeposit from forming, yet not so strongly as to slow down the depositionrate appreciably.

(c) Capable of mplrlly chclating aluminum at the interface betweenaluminum and the zincatlng solution.

Contrary to what might be expected by one skilled in the art, i havefound that the presence of good complexing agents for aluminum and zinc,such as fluoride and cyanide, is actually deleterious. it is conceivablethat this is related to the fact that the min at which such complexesform, and dissociate, is relatively slow.

Also, contrary to what might be expected by one skilled in the art, lhave found that it is not necessary to incorporate other metal ions asactivators (such as iron, copper, tin or lead). Thus, my invention ischaracterized by the use of a dilute (preferably 180 g./i. or less oftotal salts) zincate formulation containing no complexing agents forzinc or aluminum, other than hydroxide, no metal ions other than zinc,and which contains chclating agents for both zinc and aluminum, eithersingly or in combination, said chelating agents having the property ofcontrolling the activity of zinc and aluminum ions in the zincatlng bathso as to permit deposition of a metallurgieaily sound zinc deposit, yetpermitting a sufliclently rapid deposition rate so as to avoid unsounddeposits due to oxide film formation on the surface of the aluminum.

A further property of the formulations forming my invention is theability to plate a sound zine deposit on virtually all commercialaluminum alloys by suitable adiustment of (a) bath concentration,preferably in the range of 60-lti0 g./i. total salt content, (b)temperature, preferably in the range of 45'-200' F., and (c) time ofimmersion, preferably in the range of 5 seconds to 2 minutes. Theformulations which constitute my invention may be used in combinationwith my conventional treatment cycle for preparing the aluminum surface.By such conventional treatments, i understand use of etching ornon-etching (slllcated or otherwise) alkaline cleaners for aluminum;pickles, or etehants, such as those containing nitric acid;nitric-sulfuric acids: nltrlc-sulfurlc-hydro iluorlc acids (this beingan illustrative, and not a re- Oil Oil

4 strictive listing); bright dips for aluminum, either acid or alkaline;use of either a single, double, or triple dip in the zincating bath,with intermediate removal of the immersion coating in any of the picklesor bright dips mentioned above.

SCOPE OF DISCLOSURE In the tables which follow, the data are taken from(a) "Tables of Stability Constants (2 volumes), 1. Bjerrum, G.Schwartzenbach, L. D. Sillen, IUPAC (Chem. Soc., London, 1957); (b) TheSequestration of Metals," R. L. Smith (Macmillan, 1959); (e) "TheChemistry of the Coordination Compounds," J. C. Bailar, Jr., Ed., A.C.S.(Reinhold, i956) and (d) "Chemistry of the Metal Chclate Compounds," A.E. Martell, and M. Calvin (Prentice-Hall, 1952).

The numbers given below (unless otherwise stated) are for the commonlogarithm of the stability constant, k defined as:

where 1 denotes activity, M is the metal ion involved, and X is thechelate ion involved. in a few cases, other k values are indicated:

The cheiates which I discovered to be effective (as additives to thebasic dilute zlncate formulations) may be grouped as follows:

These will now be taken up in detail. The tables which are included areillustrative and not restrictive. The tables are largely, though notexclusively, limited to compounds for which stability constant data areavailable in the above-mentioned references.

1. AMINES A. Mon0mnlncs.-l have discovered as one class of chclatingagents, having a log k, zinc stability constant of 4.5 to l8, the use ofmono-amines of the general formula Its-N-R where hydrogen constitutes0-2 of the R members, and the balance of the R members contain 1-4carbons in each R member and at least one of the R members con- 7 8 C.Poiyamincs.1 also discovered as having similar Table IlACnti.nuedchelating action as Classes IA and 1B, the use of polyamincs of thegeneral formula Name Formula 10 :1 Al

5 Clluconate COO- 1.7

Rt Rs CHOU) l"/ IL CH --NR (011) N 0 1: l J. Us H Baoehsrata CO0- CHOU)where hydrogen constitutes 0-4 of the R members, the 000- balance of theR members each containing 1-4 carbons, H at least one of the R memberscontaining a hydroxyor (m l no earboxylic grouping; further, wherein Ris either hydrogen or an organic group as in the case of R members; IIfurther, wherein x is 1-4. Examples are given in the following Table 16no Table Name Formula 1 k M 0Ae\ AeO Dlathylenatrlmulne pentsaretateN-(Clls)sN-(Cl1|)sN 15.1

OAc e0 AeO Trlethyisneletramine N1h-l(Ciislr-Nllls-(Cllsh-Nlis l2.0i'entnethylenellcsemlne Nll|-l(Cl1slr-Nills-(Cihlg-Nlls 10.2

ll. CARilOXYLiC ACiD OR CORRESPONDING SALT A. a-ffydroxy cnrboxylie.lhave found that these compounds having a log k, zinc stability constantof about 1.5 to 4, particularly e-hydroxy cnrboxylic acids (or theirsalts) of the general formula lit 110-(5-0 0 011 where hydrogenconstitutes 0-2 of the R members, and the balance of the R memberscontain 1-4 carbons in combination with cheiating agents of Classes 1A,1B, 1C and 111A function to activate the zlncating process. Suchcarbon-containing R members may be hydrocarbons, such as alkyl groups,but preferably contain oxygen, such as earboxylic, aldehydic, orhydroxyl groups. Examples are given in the following Table A B.Dlcarboxylle.-Thesc compounds having a similar log k; zinc stabilityconstant as Class "A, namely, dicarboxylic acids (or their salts), ofthe general formula where R contains 0-2 carbons likewise functioneffectively in the combination. Examples are given in the following C.aand p-Keto CarboxyIlc.-The use of a-lteto carboxyiic acids (or theirsalts) of the general formula wherein R contains 1-3 carbons in a chain,with or without side chains in the combination produces novel andunexpected chelating results. While not necessary it is preferable thatcarbons in the 0-position to the CO group in the formula above containhydroxy or carboxylic groupings. I further claim fi-keto carboxylicacids (or their salts) of the general formula RCOCH -COOH wherein R isdefined as above. Examples are given in the following table iii. OTHEROXY-COMPOUNDS A. a-llyrlmx) aivoltol.r.'lhcse compounds having a similarlog k, zinc stability constant as Classes "A and HB. namely, the use ofa-hydroxy alcohols of the general formula wherein R is preferablyhydrogen, but may also represent a chain containing l-3 carbons; whereinR may be hydrogen, or may represent a chain of 1-4 carbons, said carbonspreferably. but not necessarily, containing hydroxyi groupings arelikewise effective in the combination. Examples are given in thefollowing table Table IIIA Name Formula Propylene glycol (llyenrolMnuuitoi: florlritol B. {J-I)Iketm's.-These compounds. namely,[Milketones of the general formula a,-co-cu,-co-a,

where R, and R, have 1-3 carbons, with or without side chains have asimilar cheinting action in the combination as Classes "A, ill! and "C.An example is given in the following table iii) Table 1118 Name Formulalog k Al Acetylaeetone CIh-CO-Cih-CO-Clh 6.1 &0

Urn-9.0. it-HMS; kin-22.3.

C. a-Hydroxy aryi compounds-These compounds.

namely, the use of a-hydroxy aryi compounds of the general formula whereR is a side chain containing oxygen, such as OH. -CHO, COOH have thesame cheiating effect in the combination as Classes IA, iB, IC and 1118.In addition, other substituents may be present on other positions in thearyl ring, such as SO;,H, -Ci, NO,, etc. Such substituents do notcontribute to any essential cheiating feature, but may contribute otherdesirable features, such as greater solubility in water. 1 further claimsubstituted phenols of the general formula wherein R is as definedabove, and R is an easily hydrolyzed grouping, such as a cnrboxylgrouping. Examples are given in the following table Table INC NameFormula lo; in Al (lataehoi -0il -Oll Ballot-late -0lt H BuliosalleylateOil (land -0|B- COO- 0Cl0(llis Amlylsalleyiate lt 0ll 8ni|aylaldaltylo.......... LA -Cii0 Oli Bulfasalloyinldnltyrla tLO -0 B -Oll0ins=i'l.4.

D. Ailzarln derlvallvcs.-Sucit compounds, namely. the use of aiizarinand its derivatives have similar cheiating action in the combination asClass lIlC. Examples are given in the following table Table IIID NameFormula Al Allsarln -80s" Ailsarln Sultanate Oll An examination of thetables will show that relatively little data are available for aluminum,but somewhat more for zine. it will be noted that the stabilityconstants for zinc cheiates fall into two broad categories:

(a) Log k, is 45-18 in Tables I and iiiB-D (b) Log k, is 1.5-4 in Tableii and liiA Qualitative data in the literature indicate a similarclasslficatlon for aluminum. There are no quantitative data for TableliiA, but qualitative data from the literature suggest theclassification given above.

On the basis of the classification given above, it might at first seemthat the chelating agents of Tables i and lilll-i) would be superior tothose of Tables ii and HM. However, i have discovered a synergisticrelationship between the two above classifications of chelating agentswhich would not be evident to one skilled in the art of either chelatingagents in deposition solutions or of plating on aluminum.

Accordingly, in addition to the agents referred to prevlousiy, andsubject to limitations imposed elsewhere, the use of any of thefollowing classes of chelating agents: 1A, 1B, lC, lliB, lllC, iliD (asheretofore defined), singly or in combination, in conjunction with theuse of any of the following classes of chelating agents: "A; 11B; 11C; M(as heretofore defined), singly or in combination. i have discovered thesurprising and unexpected resuit that zincating formulations containingthe two above classes of chelating agents coniunctiveiy are moreefficacious than those employing chelating agents drawn from only one ofthe above two classifications.

i believe that the synergistic action of a combination of chelatingingredients producing the desired zincatlng action is unpredictable andwould hypothesize this action, without restriction as to the scope ofthe specification and claims as follows:

"Zlncating" of aluminum surfaces is a galvanic displacement reaction,wherein zinc ions are reduced to zinc metal, and aluminum is oxidized toaluminum ions. If the free ions were employed (as in acid solutions, forexample), the resulting galvanic deposits would be dendritic (i.e.,trced." of coarse, and non-coherent erystalline structure). Thus it isnecessary to complex or chetill late the ions involved. The complexingagent most com monly employed is hydroxide, i.e., highly alkalinesolutions are employed. The stability constants for hydroxide complexesare: log k =i4.9 (Zn); 33.8 (Al). Thus, in theory, the so-cailedzincating" of aluminum surfaces would take place by the followingoxidation-reduction reaction:

in practice, however, the rate at which such hydroxide complexes form isslow. Thus, if the "zineating" reaction is too fast, aluminum hydroxidewill precipitate on the surface and be incorporated with the zincdeposit, leading to unsatisfactory results. A simple "zincating"formulation, i.e., Na0H+a zinc salt, is satisfactory for conventionalconcentrated zineating baths. However, the dilute baths operate morerapidly, so rapidly indeed, that hydroxide precipitates form on thesurface. Thus, in dilute zincaling formulations, it is necessary toemploy cheiating agents which will rapidly cheiate aluminum (to preventprecipitate formation), and which will rapidly liberate zinc ions (topermit the oxidation-reduction reaction to proceed in the first place).it would appear (based on practical results obtained, but not on kineticor other rate-indicative data) that cheiates of groups II and "IA permitsuch rapid chelating action. However, their use alone is not the optimumsolution of the problem, since the stability constants are low, thusleading to the eventual precipitation of hydroxides somewhere in thegeneral vicinity of the aluminum surface. Convection and diffusion inthe solution will then lead to re-deposlt such hydroxides on the surfacebeing plated. By also adding cheiates of types 1 and lllB-D, thisproblem is overcome since these have much higher stability constants.However, when cheiates of the latter types are used alone, less thanoptimum results are obtained since their rate of cheiate formation withAl, and liberation of Zn ions is slower (based on practicalobservations, and not on kinetic data). Thus, there is some risk ofhydroxide precipitate formation directly on the surface of the work.While hypothetical, the above model would account for the unexpectedsynergistic effects noted when both classes of chelating agents arepresent.

The chelating agents of the present invention are preferably used infa'muiations containing about 60-180 g./i. of total salts. Of the totalsalt content, the following are present in the amounts indicated:

(a) An alkali metal hydroxide, preferably sodium hydroxide, about 60 toparts by weight.

(b) A zine salt (such as zinc oxide, zinc sulfate, etc.) about 5.5 to 12parts by weight based on the zinc content. A preferred economical saltis zinc oxide.

(0) Weight ratio of OH-IZn (as metal): about 2.1- 7.9 (expressed asNaOH/ZnO: 4-15).

(d) A chelating reagent in an amount of from about 5 to 20 parts byweight comprising the combination of (i) from about 5 to percent of atleast one water soluble chelating agent having a log k, zinc stabilityconstant of about 4.5 to 18 and (2) from about 95 to 5 percent of atleast one water soluble chelating agent having a 'log k; zinc stabilityconstant of about L5 to 4. (3) Optionally, surface active agents may bepresent to lower surface tension up to about 2 parts by weight.

The examples of formulas corresponding to the above set forth in thefollowing table are purely illustrative and not to be regarded aslimiting the scope of the invention as defined in the accompanyingclaims.

Parts. (wt.)

Acetyl Aretnna. Aretyi sullcylatu (Na).... litulun trlacetate (Na).Catecltol Dlethylene trlantlue peutanretate (Na) Etltyleuedlnmlnetetraaretate n) Ethylene gl col. (llucouatet a) (ilyrerol ltocitulleHalt t orhitol Waiting Agent (anionic).

The preceding examples may be used at temperatures from 45-200 F., withimmersion times of sec. to 2 min. The higher the temperature, theshorter the immersion time. The lower the total salt concentration, theshorter the immersion time. The more electrochemically active thealuminum alloy surface, the shorter the immersion time. For most commoncommercial alloys of aluminum, the optimum conditions are 70-95 F., withimmersion times of sec.-l min. As with conventional zincating practice.the zinc coating may be removed by a brief dip in nitric acid, thenre-applicd (the "double zincate" process).

in the foregoing specification and the claims to follow, the salts ofboth the basic and acidic chelating agents. including but not limited toamines and carboxylic acids, are to be regarded as equivalent to theunsubstituted acid and base derivatives.

While i have shown and described a preferred embodiment of my invention,it will be understood that it is not to be limited to all of the detailsshown. but is capable of modification and variation within the spirit ofthe invention and within the scope of the claims.

What i claim is:

i. A zincating bath for aluminum metals comprising a dilute aqueoussolution of a zinc salt having a total salt concentration of no morethan about ltiO grams per liter, an alkaline compound and a chelatingreagent capable of chelating both aluminum and 'zinc consistingessentially of the combination of (i) from about 5 to 95 percent byweight of a water soluble chelating agent having a log Ir, zinestability constant of about 4.5 to ill and (2) from about 95 to 5percent by weight of a water soluble chelating agent having a log k,zine stability constant of about 1.5 to 4.

2. A zincating bath for aluminum metals comprising a dilute aqueoussolution of a zinc salt having a total salt concentration of from about60 to tilt) grams per liter. an alkaline compound and a chelatingreagent capable of chelating both aluminum and zinc consistingessentially of the combination of (i) from about 5 to 95 percent byweight of a water soluble chelating agent having a log k, zinc stabilityconstant of about 4.5 to lit and (2) from about 95 to 5 percent byweight of a water soluble chelating agent having a log k, zinc stabilityconstant of about L5 to 4.

3. A zincating bath for preparing aluminum metals for plating comprisinga dilute aqueous solution of a zinc salt, an alkaline compound and achelating reagent comprising, in combination, (i) from about 5 to 95percent by weight of a compound having a log k, stability constant forzinc of about 4.5 to ill and selected from the class consisting of themonoamines. polyamines and their salts l'i-dikctoncs, a-hydroxyarylcompounds and aiizarin derivatives and their salts, and (2) from about95 to 5 percent by weight of a compound having a log k, stabilityconstant for zinc of about L5 to 4 and selected from the classconsisting of a-hydroxy carboxylic acids and their salts, dicarboxylicacids and their salts, aand fl-keto carboxylic acids and their salts anda-hydroxy alcohols.

4. A zincating bath for aluminum metals as set forth in claim 3, whereinthe total salt concentration is approximately 60 to 180 grams per liter.

5. A zincating bath for aluminum metals as set forth in claim 3, whereinthe chelating reagent is the combination of an amine and an a-hydroxycarboxylic acid.

6. A zincating bath for aluminum metals as set forth in claim 3, whereinthe chelating reagent is the combination of an amine and an m-hydroxyalcohol.

7. A zincating bath for aluminum metals as set forth in claim 3, whereinthe chelating reagent is the combination of an a-hydroxyaryl compoundand an a-hydroxy carboxylic acid.

8. A zincating bath for aluminum metals as set forth in claim 3, whereinthe chelating reagent is the combination of an a-hydroxyaryl compoundand an et-hydroxyi alcohol.

9. A zincating composition for preparing aluminum metal surfaces forplating comprising about 60 to parts by weight of an alkali metalhydroxide, about 7 to 15 parts by weight of a zinc salt and about 5 to20 parts by weight of a chelating reagent capable of chelating both zincand aluminum consisting essentially of, in combination, (I) from about 5to percent by weight of a chelating agent having a log k, zinc stabilityconstant of about 4.5 to 18 and (2) from about 95 to 5 percent by weightof a chelating agent having a log k, zinc stability constant of L5 to 4.

10. A zincating composition for preparing aluminum metal surfaces forplating comprising about 60 to 85 parts by weight of an alkali metalhydroxide, about 7 to l5 parts by weight of a zinc salt and about 5 to20 parts by weight of a chelating reagent capable of chelating both zincand aluminum consisting essentially of, in combination. (1) from about 5to 95 percent by weight of a chelating agent having a log k, zincstability constant of about 4.5 to l8 and (2) from about 95 to 5 percentby weight of a chelating agent having a log k, zinc stability constantof 1.5 to 4, the weight ratio of hydroxyl ions to zinc being about 2.lto 7.9.

II. A zincating composition for preparing aluminum metal surfaces forplating including in combination a zinc salt, an alkali and a mixture ofchelating agents consisting essentially of (i) from about 5 to 95percent by weight of at least one water soluble chelating agent having alog k, zinc stability constant of about 4.5 to 18 and (2) from about 95to 5 percent by weight of at least one water soluble chelating agenthaving a log k, zinc stability constant of about 1.5 to 4.

12. A zincating composition for.preparing aluminum metal surfaces forplating including in combination a zinc salt, an alkali and (i) fromabout 5 to 95 percent by weight of at least one water soluble chelatingagent having a log in zinc stability constant of about 4.5 to 18selected from the class consisting of the monoamines. polyamines andtheir salts, {s-diketones, a-hydroxyaryi compounds and aiizarinderivatives and their salts, and (2) from about 95 to 5 percent byweight of a chelating agent having a log k, zinc stability constant ofabout 1.5 to 4 selected from the class consisting of e-hydroxycarboxyiic acids and their salts, dicarboxyiic acids and their salts.aand {s-keto carboxylic acids and their salts and e-hydroxy alcohols.

13. A zincating composition as set forth in claim 12, wherein thechelating reagent is the combination of an amine and an a-hydroxycarboxylic acid.

14. A zincating composition as set forth in claim l2,

wherein the cheiating reagent is the combination of an amine and adicarboxylic acid.

15. A zincating composition as set forth in claim 12, wherein thecheiating reagent is the combination of an amine and an a-ketccarboxylic acid.

16. A zincating composition as set forth in claim 12, wherein thechelating reagent is the combination of an amine and an a-hydroxyalcohol.

17. A zincating composition as set forth in claim 12, wherein thechelating reagent is the combination of an a-hydroxynryl compound and ana-hydroxy carboxylic acid.

18. A zincating composition as set forth in claim 12, wherein thechelating reagent is the combination of an a-hydroxyaryl compound and ana-hydroxy alcohol.

19. A ziacating composition aa set forth in claim 12, wherein thechelating reagent is the combination of an aiizarin derivative and ana-hydroxy carboxylic acid.

1 6 References Cited by the Examiner UNITED STATES PATENTS 2,650,8869/1953 Zelley 106-1 2,766,138 10/1956 Talmey 106-1 2,872,346 2/1959Miller 106-1 OTHER REFERENCES Bersworth Chemical Company, Versenes",Technical Bulletin No. 2, Bersworth Chemical 00., Framingham,

Mass. 1952). Sec. 1, p. 7.

15 MORRIS LIEBMAN, Primary Examiner.

JOSEPH REBOLD, ALEXANDER H. BRODMERKEL,

Examiners.

1. A ZINCATING BATH FOR ALUMINUM METALS COMPRISING A DILUTE AQUEOUSSOLUTION OF A ZINC SALT HAVING A TOTAL SALT CONCENTRATION OF NO MORETHAN ABOUT 180 GRAMS PER LITER, AN ALKALINE COMPOUND AND A CHELATINGREAGENT CAPABLE OF CHELATING BOTH ALUMINUM AND ZINC CONSISTINGESSENTIALLY OF THE COMBINATION OF (1) FROM ABOUT 5 TO 95 PERCENT BYWEIGHT OF A WATER SOLUBLE CHELATING AGENT HAVING A LONG K1 ZINCSTABILITY CONSTANT OF ABOUT 4.5 TO 18 AND (2) FROM ABOUT 95 TO 5 PERCENTBY WEIGHT OF A WATER SOLUBLE CHELATING AGENT HAVING A LOG K1 ZINCSTABILITY CONSTANT OF ABOUT 1.5 TO 4.